1. Field of the Invention
The present invention relates to a vehicle motion control device and more specifically to a control device mounted in a four-wheel-drive vehicle having independent front wheel and rear wheel drive assemblies.
2. Description of the Related Art
It is known that when a vehicle is accelerated, decelerated, or turned, various vibrations can occur in the vehicle. Specifically, when a vehicle driver provides a motion command to the vehicle, for example by depressing the accelerator pedal, unnecessary motion of a vehicle body, such as a pitching motion, a vertical motion, or vibrations of tires can be made by the vehicle in addition to a motion associated with the running command.
Conventional technology, as described, for example, in JP-A-2006-60936, has been developed to prevent some unnecessary motion. In the described technology, a larger amount of engine energy than that actually required is generated. When an amount of energy associated with the unnecessary motion, such as pitching vibration, becomes positive, a motor generator is used as a generator to remove the pitching vibration energy suppressing the unnecessary motion. Moreover, when the amount of engine energy does not reach the required amount and the amount of pitching vibration energy becomes negative, the motor generator is used as a motor to supplement the engine energy removing the pitching vibration energy. Through the above actions, deterioration in vehicle behavior associated with the pitching vibration energy is prevented to stabilize vehicle motion.
A conventional vehicle motion control device is also known, as described below. Specifically, the vehicle motion control device is mounted in an electric four-wheel-drive vehicle the front wheels of which are driven by inducing a torque in an output shaft for the front wheels using an internal combustion engine, and the rear wheels of which are driven by inducing a torque in an output shaft for the rear wheels using a motor generator. On a so-called “low μ road,” which is a road having a relatively low coefficient of friction against tires, when the vehicle is started in a low velocity band of, for example, 30 km/hour or less, a torque is induced in the output shaft for the rear wheels using the motor generator. By thus delivering a driving force to the rear wheels, a facility for assisting in start of the vehicle is realized.
When the unnecessary motion causes an oscillation in a load acting on tires, or especially, when the vehicle motion causes a body vibration in vertical direction, or during one of many other various factors causes unnecessary motion, vibration or movement, a problem arises as will be described in greater detail below.
Specifically, among forces capable of being transmitted to a road via tires, a frictional force generated between the tires and road has the largest magnitude. The magnitude of the frictional force is determined as a product of a load acting on the tires and a coefficient of friction. Therefore, if the load acting on the tires oscillates, the frictional force generated between the tires and road also oscillates meaning that the largest force to be transmitted to the road via the tires fluctuates. When the load acting on the tires does not oscillate, the frictional force can be entirely transmitted to the road via the tires. However, if the load acting on the tires oscillates, a part of the force may not be transmitted to the road. In such a situation, even when a torque can be faithfully induced in an output shaft via a drive assembly including an internal combustion engine or the like in response to a request made by a driver, the output may not be properly transmitted to the road via the tires and the acceleration of the vehicle may be disturbed. Thus, the unnecessary motion may bring about a disturbance in the acceleration of the vehicle and vehicle motion meeting the intention of the driver becomes difficult to achieve.